A CRT is evacuated to a very low internal pressure and accordingly is subjected to the possibility of implosion due to the stresses produced by atmospheric pressure acting on all surfaces of the tube. This problem has been addressed in the art by providing the CRT with an implosion protection band. Such a band is used to apply a compressive force to the sidewall of the CRT to redistribute some of the faceplate forces. The redistribution of the faceplate forces decreases the probability of an implosion of the tube by minimizing tension in the corners of the faceplate. An implosion protection band is also beneficial because it improves the impact resistance of the tube. Glass in compression is stronger than glass which is in tension and the band causes compression in faceplate areas which otherwise would be in tension. Additionally, in the event of an implosion, the redistributed stresses cause the imploding glass to be directed toward the back of the cabinet in which the tube is mounted, thereby substantially reducing the probability of someone in the vicinity of the imploding tube being injured.
Mounting lugs, either integral with, attached to, or disposed between the band and the tube sidewall are used to support the tube within the cabinet. Typically, the mounting lugs are positioned at the corners of the tube and aligned along the faceplate diagonals, although other placements of the mounting lugs are known. One drawback of such lugs is that as CRT's are produced in larger sizes, especially with diagonal dimensions in excess of 75 cm, the tube weight increases and puts considerable stress on the mounting lugs. Lugs which are attached to the surface of the implosion protection band, for example by welding, are prone to failure unless the weld is carefully made; however, it is difficult to inspect the quality of such welds without destructively testing the welded lug-band assembly. To overcome this problem, it is known to manufacture bands with integral lugs. One such structure is shown in U.S. Pat No. 4,295,574 issued to Nakazima et al. on Oct. 20, 1981. The patent discloses a shrinkfit band, formed flat, having integral lugs positioned along the band so that the lugs will be located at the corners of the tube, when the band is attached to the tube and the lugs are bent out of the plane of the band. A drawback of such a structure is that since the lugs must be bent out of the plane of the band, the material must be soft enough to permit the bending; however, the mechanical strength of such integral lugs is suspect, when used for heavy, large size tubes.
U.S. Pat. No. 5,055,934, issued to H. R. Swank on Oct. 8, 1991 overcomes the aforementioned problems by positioning the lugs between the band and the tube sidewall, within concavities formed in the band. The lugs are then secured to the overlying band. The lugs can be fashioned of material suitable for supporting even the largest tubes. One drawback of this structure is that the use of separate lugs increases the cost of the tube assembly over tube assemblies using bands in which the lugs are integral therewith.
A need thus exists for an implosion prevention structure having the cost effectiveness of a band with integral lugs but having structural integrity sufficient to support the present types of large tubes.